For terrestrial television (TV), signals can exist in any of multiple bands, namely very high frequency (VHF)-low, VHF-high, and ultra high frequency (UHF), collectively ranging from 54 MHz to 806 MHz, while for cable TV, signals can exist anywhere from 54 MHz to 1002 MHz. Thus a given television tuner must be designed to handle this extremely large frequency range. The tuner receives incoming signals at these high frequencies and downconverts them to a lower frequency, generally referred to as an intermediate frequency (IF) for certain signal processing. These downconverted signals are still further downconverted to baseband for further processing and demodulation to obtain audio and visual signals for output to an associated display.
To effect the downconversion, a mixer is provided that receives the incoming radio frequency (RF) signal and mixes it with a local oscillator (LO) frequency to obtain a lower frequency signal. To avoid interference with harmonics of this LO signal, a harmonic rejection mixer may be used. In U.S. Pat. No. 7,756,504, the disclosure of which is hereby incorporated by reference, a rotating harmonic rejection mixer was disclosed, where to further reduce harmonics, the output signal of a mixer is switched among multiple IF stages, the outputs of which are summed together to generate an IF signal for further processing. The number of stages to which the IF signal is rotated can vary based upon the input signal. The rotating harmonic rejection mixer has a property of rejecting all harmonics until N-1, where N is the number of rotation stages used. While the disclosed rotating harmonic rejection mixer represents an improvement over existing harmonic rejection mixers, challenges still exist in implementing the mixer into a feasible semiconductor device.
Many receivers implement other mixer types such as a Gilbert cell type quadrature mixer. However, such mixers are noisy for given current and linearity requirements. Accordingly, receivers generally include various front end RF circuitry in front of a mixer to reduce noise. Particularly, receivers generally incorporate a low noise amplifier (LNA) to both amplify an incoming RF signal and to reduce its noise factor prior to input to the mixer. Oftentimes, this low noise amplifier can be of a differential form, as the incoming RF signal to the LNA may be differentially provided, e.g., as output from a balun, which is a circuit to receive a single-ended RF signal and convert it to differential form. Typically, the balun can be used to provide a balanced input to the LNA, aiding in a higher second order linearity (IIP2) of the receiver. Oftentimes further included in this front end circuitry can be some type of filter such as a pre-filter, tracking filter or so forth. Generally the RF filter can be used to attenuate blockers and improve overall system selectivity and further aid in harmonic rejection performance by attenuating blockers around the LO frequency.
However, by imposition of these front end components, overall system linearity can be degraded as the mixer receives amplified signals. And, the inclusion of these front end circuits increase circuit complexity, cost, power consumption and chip real estate.